1. Field of the Invention
This invention relates generally to improvements in infusion pumps such as those used for controlled delivery of medication to a patient. More specifically, this invention relates to improved methods and apparatuses for detecting errors in detecting fluid pressure and occlusions in fluid delivery paths of infusion pump systems.
2. Description of Related Art
Infusion pump devices and systems are relatively well-known in the medical arts, for use in delivering or dispensing a prescribed medication such as insulin to a patient. In one form, such devices comprise a relatively compact pump housing adapted to receive a syringe or reservoir carrying a prescribed medication for administration to the patient through infusion tubing and an associated catheter or infusion set.
The infusion pump includes a small drive motor connected via a lead screw assembly for motor-driven advancement of a reservoir piston to administer the medication to the user. Programmable controls can operate the drive motor continuously or at periodic intervals to obtain a closely controlled and accurate delivery of the medication over an extended period of time. Such infusion pumps are used to administer insulin and other medications, with exemplary pump constructions being shown and described in U.S. Pat. Nos. 4,562,751; 4,678,408; 4,685,903; 5,080,653 and 5,097,122, which are incorporated by reference herein.
Infusion pumps of the general type described above have provided significant advantages and benefits with respect to accurate delivery of medication or other fluids over an extended period of time. The infusion pump can be designed to be extremely compact as well as water resistant, and may thus be adapted to be carried by the user, for example, by means of a belt clip or the like. As a result, important medication can be delivered to the user with precision and in an automated manner, without significant restriction on the user's mobility or life-style, including in some cases the ability to participate in water sports.
These pumps often incorporate drive systems which use a lead screw coupled to motors. The motors can be of the DC, stepper or solenoid varieties. These drive systems provide an axial displacement of the syringe or reservoir piston thereby dispensing the medication to the user. Powered drive systems are advantageous since they can be electronically controlled to deliver a predetermined amount of medication by means well known in the art.
In the operation of these pump systems, the reservoir piston will be fully advanced when virtually all of the fluid in the reservoir has been dispensed. Correspondingly, the axial displacement of the motor lead screw is also typically fully displaced. In order to insert a new reservoir, which is full of fluid, it is necessary to restore the lead screw to its original position. Thus the lead screw will have to be rewound or reset.
DC motors and stepper motors are advantageous over solenoid motors in that the former are typically easier to operate at speeds that allow rewinding the drive system electronically. Solenoid based drive systems, on the other hand, often must be reset manually, which in turn makes water resistant construction of the pump housing more difficult.
Lead screw drive systems commonly use several gears which are external to the motor. FIG. 1 shows such a lead screw arrangement which is known in the art. A motor 101 drives a lead screw 102 which has threads which are engaged with a drive nut 103. Thus the rotational force of the lead screw 102 is transferred to the drive nut 103 which causes it to move in an axial direction d. Because the drive nut 103 is fixably attached to a reservoir piston 104 by a latch arm 110, it likewise will be forced in an axial direction d′, parallel to direction d, thus dispensing the fluid from a reservoir 105 into an infusion set 106. The lead screw 102 is mounted on a bearing which provides lateral support. The lead screw 102 extends through the bearing and comes in contact with the occlusion detector. One known detector uses an “on/off” pressure limit switch.
Should an occlusion arise in the infusion set 106 tubing, a back pressure will build up in the reservoir 105 as the piston 104 attempts to advance. The force of the piston 104 pushing against the increased back pressure will result in an axial force of the lead screw 102 driving against the detector. If the detector is a pressure limit switch, then an axial force that exceeds the set point of the pressure limit switch will cause the switch to close thus providing an electrical signal through electrical leads and to the system's electronics. This, in turn, can provide a system alarm. The entire assembly can be contained in a water resistant housing 107.
FIG. 2 shows a different drive system and lead screw arrangement which also is known in the art. In this arrangement, a motor 201 (or a motor with an attached gear box) has a drive shaft 201 a which drives a set of gears 202. The torque is then transferred from the gears 202 to a lead screw 203. The threads of the lead screw 203 are engaged with threads [not shown] in a plunger slide 204. Thus the torque of the lead screw 203 is transferred to the slide 204 which causes it to move in an axial direction d′, parallel to the drive shaft 201 a of the motor 201. The slide 204 is in contact with a reservoir piston 205 which likewise will be forced to travel in the axial direction d′ thus dispensing fluid from a reservoir 206 into an infusion set 207. The lead screw 203 is mounted on a bearing 209 which provides lateral support. The lead screw 203 can extend through the bearing to come in contact with an occlusion detector. As before, if the detector is a pressure limit switch, then an axial force that exceeds the set point of the pressure limit switch will cause the switch to close thus providing an electrical signal through electrical leads and to the system's electronics. This, in turn, can provide a system alarm. The assembly can be contained in a water resistant housing 208.
As previously noted, these lead screw drive systems use gears which are external to the motor. The gears are in combination with a lead screw with external threads which are used to drive the reservoir's piston. This external arrangement occupies a substantial volume which can increase the overall size of the pump. Moreover, as the number of drive components, such as gears and lead screw, increases, the torque required to overcome inherent mechanical inefficiencies can also increase. As a result, a motor having sufficient torque also often has a consequent demand for increased electrical power.
Yet another known drive is depicted in FIGS. 3a and 3b. A reservoir 301 fits into the unit's housing 302. Also shown are the piston member 303 which is comprised of an elongated member with a substantially circular piston head 304 for displacing the fluid in the reservoir 301 when driven by the rotating drive screw 305 on the shaft (not visible) of the drive motor 306.
As is more clearly shown in FIG. 3b, the reservoir 301, piston head 304 and piston member 303 comprise an integrated unit which is placed into the housing 302 (FIG. 3a). The circular piston head 304 displaces fluid in the reservoir upon axial motion of the piston member 303. The rearward portion of the piston member 303 is shaped like a longitudinal segment of a cylinder as shown in FIG. 3b and is internally threaded so that it may be inserted into a position of engagement with the drive screw 305. The drive screw 305 is a threaded screw gear of a diameter to mesh with the internal threads of the piston member 303. Thus the motor 306 rotates the drive screw 305 which engages the threads of the piston member 303 to displace the piston head 304 in an axial direction d.
While the in-line drive system of FIG. 3a achieves a more compact physical pump size, there are problems associated with the design. The reservoir, piston head and threaded piston member constitute an integrated unit. Thus when the medication is depleted, the unit must be replaced. This results in a relatively expensive disposable item due to the number of components which go into its construction.
Moreover the drive screw 305 and piston head 304 of FIG. 3a are not water resistant. Because the reservoir, piston head and threaded piston member are removable, the drive screw 305 is exposed to the atmosphere. Any water which might come in contact with the drive screw 305 may result in corrosion or contamination which would affect performance or result in drive failure.
The design of FIG. 3a further gives rise to problems associated with position detection of the piston head 304. The piston member 303 can be decoupled from the drive screw 305. However, when another reservoir assembly is inserted, it is not known by the system whether the piston head 304 is in the fully retracted position or in some intermediate position. Complications therefore are presented with respect to providing an ability to electronically detect the position of the piston head 304 in order to determine the extent to which the medication in reservoir 301 has been depleted.
The construction of pumps to be water resistant can give rise to operational problems. As the user travels from various elevations, such as might occur when traveling in an airplane, or as the user engages in other activities which expose the pump to changing atmospheric pressures, differential pressures can arise between the interior of the air tight/water-resistant pump housing and the atmosphere. Should the pressure in the housing exceed external atmospheric pressure, the resulting forces could cause the reservoir piston to be driven inward thus delivering unwanted medication.
Thus it is desirable to have an improved, compact, water resistant drive system which permits safe user activity among various atmospheric pressures and other operating conditions. Moreover it is desirable to have improved medication reservoir pistons for use with such drive systems.